Chronic low-frequency stimulation was used for studying the adaptive potential of rat fast-twitch muscle to increased neuromuscular activity. The sequential exchange of myosin heavy chain isoforms HCIIb with HCIId and HCIIa was studied at the translational level using an in-vivo-labeling technique with [35S]methionine. Alterations in heavy chain isoform synthesis, i.e. a decrease in the labeling of HCIIb concomitant with an enhanced labeling of HCIId/IIa, were detectable already two days after the onset of stimulation. This time course corresponds to the previously observed alterations in the amounts of HCIIb and HCIIa mRNAs. However, significant changes in the relative protein amounts of HCIIb and HCIId/IIa were recorded only after an 8-day stimulation period. This delay at the protein level was interpreted to relate to the slow turnover of HCIIb which was estimated from its decay in long-term stimulated muscles with an approximate value of 14.7 days. Therefore, protein degradation seems to be an important post-translational regulatory step in the remodeling process of the thick filament.